Direct numerical simulation of frictional drag modulation in horizontal channel flow subjected to single large-sized bubble injection

Abstract

Bubble drag reduction (BDR) is desirable for achieving better propulsion performance in underwater applications. Large-sized bubbles have significant potential for application in BDR, as they provide excellent drag reduction. In this study, we analyzed the drag modulation effects of large-sized bubbles, especially high-Weber number bubbles, on the horizontal turbulent channel flow using direct numerical simulation. The Weber number (We) was varied from 130 to 337 based on the equivalent diameter and bubble velocity. The two-phase model based on the volume-of-fluid approach and IsoAdvector method for sharpening the bubble interface was used. To study the drag modulation mechanism of large-sized bubble, the skin friction profile along the centerline of the bubble and local mean ratio of skin friction were analyzed at different Weber numbers. The bubble length was found to have a significant effect on drag reduction toward the rear region of the bubble. Based on this observation, we analyzed the influence of the bubble regions on drag modulation by confining each region. The numerical results indicated that the skin friction was sensitive to the bubble size and bubble regions, the changes in the skin friction profile, contour, and local averaged values with the variation in the Weber number were analyzed. Considering the relationship between the bubble regions and skin friction, most bubbles exhibited an increase in drag on the liquid film until We = 337. Meanwhile, the area of drag reduction in the secondary flow increased and broadened with an increasing Weber number. Owing to this tendency, the effect of drag reduction is expected from sufficiently larger bubbles around We = 337. These results afford new insights into bubble-induced drag modulation in different regions of horizontal flow, and important parameters of large-sized bubbly flow for the investigation of overall drag reduction performance.